Multilevel converters are of interest to use in a number of different power transmission environments. They may for instance be used as voltage source converters in direct current power transmission systems. They may also be used as reactive compensation circuits such as Static VAR compensators. The multilevel converter is then typically based on a number of converter cells, where a cell essentially provides a voltage contribution that is either zero or based on the voltage of an energy storage element of the cell, such as the voltage across a cell capacitor.
In the operation of these multilevel converters there is typically provided a reference voltage which the cells are together switched to resemble. Furthermore, in operation the cells will typically be charged or discharged. In order to allow this charging and discharging the cells may furthermore operate in a cell voltage range that is bounded by an upper and a lower cell voltage boundary. A cell is typically not allowed to have a voltage above the upper cell voltage boundary or below the lower cell voltage boundary.
In this switching the cells are thus switched both for obtaining the reference voltage and in order for the cell voltage not to go outside of the cell voltage range, which latter control is also termed cell voltage balancing. However, switching is associated with losses as well as other undesirable phenomena such as the causing of ripple. It is therefore of interest to reduce the switching
One way of reducing the switching for load balancing purposes is described in CN102130619A1, where cells with highest and lowest cell voltages are selected based on the current direction.
There is however still a need for improvement in the way that cells are switched in a multilevel converter in order to reduce the switching losses.